The present invention relates to a process of producing chlorine dioxide involving formation of solid acidic alkali metal sulfate as a by-product. The process comprises a step of decreasing the acidity of the alkali metal sulfate by treatment with an aqueous medium.
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced.
There are numerous different processes for chlorine dioxide production. Most processes in commercial use involves continuous reaction of alkali metal chlorate in an acidic medium with a reducing agent such as methanol, hydrogen peroxide, chloride ions or sulfur dioxide. The most efficient processes are operated at subatmospheric pressure in which the reaction medium boils and water is evaporated to dilute the chlorine dioxide formed. Generally, the acidity is provided by addition of sulfuric acid and the sulfate is withdrawn as a by-product in the form of solid alkali metal sulfate, normally sodium sulfate. If methanol is used as the reducing agent, it is possible to produce chlorine dioxide without formation or with very low formation of chlorine as a by-product. However, in order to achieve satisfactory production rate, it is necessary to operate the process at a high acidity, causing also the by-product alkali metal sulfate to be acidic. Thus, in the most commonly used subatmospheric processes operated with methanol as the reducing agent, such as in those described in the U.S. Pat. No. 4,081,520 or EP patent 445493, the solid by-product formed is acidic sodium sulfate, normally sodium sesqui sulfate. The acidic sodium sulfate normally has to be neutralized which causes consumption of caustic. Further, the acid lost with the acidic sulfate has to be replaced by supplying more acid to the chlorine dioxide reactor.
WO 94/11300 describes a method of neutralizing acidic sodium sulfate by treatment with an aqueous medium to effect conversion of the solid material to neutral sodium sulfate in a mixing tank and removing the solid material by filtration. The resulting acid aqueous medium can then be recirculated to the chlorine dioxide reactor. Similar processes are described in the U.S. Pat. Nos. 4,325,934 and 5,116,595.
However, it has been found difficult to operate the prior art processes in full scale without frequent interruptions. Particularly, fluctuations in the flow from the mixing tank and in the concentration of solid material therein causes clogging in the equipment for filtration.
It is an object of the present invention to provide a process of producing chlorine dioxide involving formation of acidic alkali metal sulfate as a by-product and neutralization of said acidic alkali metal sulfate, which process can be operated without frequent interruptions. The process should be simple and easy to operate without requiring too complicated and expensive equipment.
The invention concerns a process of producing chlorine dioxide comprising the steps of reducing chlorate ions in an acid reaction medium maintained in a reaction zone of a chlorine dioxide generator, which reaction medium contains alkali metal ions and sulfate ions, so to form chlorine dioxide and a solid salt of acidic alkali metal sulfate. The process further comprises the steps of separating said solid acidic alkali metal sulfate from the reaction medium, contacting said solid acidic alkali metal sulfate with an preferably acid aqueous medium in a mixing tank to effect at least partial conversion to neutral solid alkali metal sulfate, separating the at least partially neutralized solid alkali metal sulfate from the aqueous medium on a rotary drum filter operating with vacuum inside and with part of the drum submerged in a slurry of the at least partially neutralized solid alkali metal sulfate and the aqueous medium, and forming an acid filtrate.